Random reactor packings provide good heat transfer between a reactor wall and fluid passing through the reactor. The particles in the packing randomly direct the fluid to flow in various directions, including impingement upon the reactor wall. Such impingement results in an increase in the heat transfer coefficient across the boundary layer at the reactor wall. The diversion of fluid to impinge the wall of a reactor while the bulk of the fluid generally flows parallel to the wall of the reactor is most effective when the packing is closest to, or preferably in contact with, the wall, as this interrupts fluid from flowing parallel to the wall, which would otherwise cause the heat transfer coefficient across the boundary layer to be relatively low.
Random packings have the advantage over structured packings in that the particles are free to move relative to each other to fill gaps between the particles and between the particles and the reactor wall. Such gaps are undesirable in that they lower the coefficient of heat transfer between the reactor wall and fluid within the reactor.
Structured packings have advantages over random packings in that they may have a higher void volume than random packings. Such higher void volume associated with structured packings results in a lower pressure drop. Structured packings can also be designed to direct fluid to flow in the most advantageous directions for enhancement of heat transfer between the reactor and its environment. Such advantageous directions are, e.g., normal to the reactor wall, towards the wall to impinge upon it, or away from the wall to balance the mass flow to and from the wall.
Structured packings can be designed to have a certain distance, space or gap between the packing and the inner reactor wall to facilitate insertion of the packing into the reactor. Structured packings designed to have such gaps may incorporate separate internal mechanical devices to move in an outward radial direction to force the outer portion of the structured packing to move toward the reactor wall. Structured packings also are more prone to separate from the reactor wall, thereby resulting in a lower heat transfer coefficient through the boundary layer at the wall than when there is no gap.